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If the fish is darting around and acting aggressive or mean, it could be toxic water quality.
Do you have your water parameters?
If not, can you do a few water changes? If the water changes seem to help, do a few more!
25% then another 25 % wait an hour if it seems to help, then do 2 more back to back water changes.
If the fish seems better then you know what's going on. Do a 50% water change and the next day do another one.
Get a test kit, or see if you can get the water tested for free at a local store. Write down the water parameters for each thing they check and tell them you need to write it down ahead of time.
What size is the tank?
FW & SW
Favorite tank right now - 72G Bowfront Wadai Tetra, Cardinal Tetras, Ghost Glass Cat Kryptopterus bicirrhis and Clown Loach Cory haven
New to FW fish? First tank in a long time or EVER??? Need some info on keeping fish?
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